Bacterial Azoreductase

Azoreductase activity, which is of exclusively bacterial origin in the lumen of the intestine, catalyzes the reduction of the azo bond to cause the formation of aromatic amines. The highly reactive intermediates and end products have been shown to be mutagenic and carcinogenic. Azo dyes are used for coloring in the food industry, and as dyes and stains in textiles and other products. Water-soluble azo dyes are degraded by the intestinal microflora in the gastrointestinal tract (62). There is a 90% correlation between carcinogenicity and mutagenicity for aromatic amines and azo dyes tested by the Ames Salmonella test (63). The need for bacterial azoreductase and nitroreductase to activate mutagens, such as azocompounds in combination with intestinal mucosal microsomal enzymes has been demonstrated (64,65).

The reduction of azocompounds by azoreductase is mediated through a free radical mechanism that produces intermediates that react with nucleic acids and proteins. The action of azoreductase on food dyes results in the release of phenyl-, and naphthyl-substituted amines. The amines generated in the lower intestine by bacterial action are probably oxidized by microsomal enzymes in the intestinal mucosa to carcinogens.

Bacterial generation of mutagens from a number of azodyes has been demonstrated. Trypan blue, a widely used biologic stain, is converted to the mutagen O-toluidine by cell free extracts of Fusobacterium, an anaerobic organism found in the large intestine (66). Ponceau 3R, another biologic stain, is reduced Fusobacterium to 2, 4, 5-trimethylaniline which is mutagenic (67). Other azo dyes that have been shown to be transformed by bacterial reduction to mutagenic or carcinogenic products are direct black 38, direct red 2, and direct blue 15. Congo red lacks mutagenic activity, however, preincubation of dye with cecal bacteria generates mutagen-positive products (68).

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